LLAVALLOL, ARGENTINA.

Instituto Fitotécnico de Santa Catalina (UNLP) and Centro de Investigaciones Genéticas (UNLP-CONICET-CIC)

Genome size in annual species of Zea. Relation with cellular parameters and altitude

--Eduardo A. Guillín (1), Lidia Poggio (1) and Carlos A. Naranjo

(1)Also affiliated with "Depto. de Ciencias Biológicas" (FCEN, UBA)

Considerable evidence has shown correlations among genome size, cellular characters and ecological parameters in maize. Rayburn (1985) found a positive correlation between DNA content (C value), heterochromatin percentage and number of C-bands per genome and a negative correlation between nuclear DNA amount and latitude as well. Bennett (1972) concluded that "there is a relationship between nuclear DNA content and minimum generation time (MGT) in herbaceous higher plants". This would be the cause of the negative DNA-latitude correlation, because bigger nucleotypes will require longer duration and/or temperature of the growing season, and so, at high latitudes (and altitudes) selection would be operating against higher DNA contents (and, as a consequence, against longer MGT's).

Genome size has also been related to total chromosome length (TCL) and volume (TCV) (Poggio et al., 1986; Bennett, 1987; Poggio and Naranjo, 1990). Such correlation has not been tested in Zea yet.

Tito et al. (TAG, in press) have also found a positive correlation between C value and C+ heterochromatin content in species of Zea, which could be indicating that higher genome sizes are a consequence, at least in part, of the presence of intragenomic supernumerary segments.

The present investigation examines genome size (DNA content, 2C), TCL and TCV in several maize lines and races collected at different altitudes. They present different vegetative periods, and a variable number of supernumerary chromosomes (B chromosomes). The same parameters have been measured in natural populations of Zea mays ssp. mexicana, Z. mays ssp. parviglumis and the only annual species from section Luxuriantes, Zea luxurians. It is the aim of the present work to add new data and discuss the correlations among genome size, vegetative period, TCL and TCV, proportion of heterochromatin measured through C-banding technique, and altitude in the taxa analysed.

Table 1 indicates DNA content (2C), expressed in pg, TCL, TCV, and altitude for the races and lines studied.

Individuals from three populations of Z. mays ssp. parviglumis at different altitudes did not present significant differences in their genome size (2C=5.76-5.88pg; F=2.10; p<0.05). On the other hand, the analysis of several individuals from two populations belonging to Z. mays ssp. mexicana collected at 2200m and 2500m respectively yielded a significant difference (2C=7.71pg and 7.10pg respectively; F=5.14; p<0.05). It was also observed, in accordance with Rayburn and Auger (1990), that there exists a negative relationship between nuclear DNA content and altitude for populations belonging to Z. mays ssp. mays: Palomero Toluqueño, collected at 2800 meters, presented a 2C value of 5.71pg, while the Tuxpeño race, 300 meters high, showed a DNA content ranging from 6.53 to 7.15pg. Thus, correlation between genome size and altitude would be limited to Z. mays ssp. mays, and cannot be extended to the whole species. It is interesting to note in this way that in the teosintes flowering would be more strongly influenced by photoperiod than in maize.

Table 1. DNA content, total length, chromosome volume and altitude in annual species of the genus Zea.
 
Taxa DNA (2C) pg X ± SE Nucleus no. (replicates) TCL (µm) X ± SE TCV (µm3) Altitude (m)
Section Zea          
Z. mays ssp. mays          
Gaspé Flint line 4.91*   68.23±0.21 30.10  
c-tester line 5.78±0.05 40(2) --- ---  
Black Mexican Race (0B) 5.63±0.14 20(1)  92.61** 41.64**  
Black Mexican Race (2B) 6.52±0.14 20(1) 99.98** 43.23**  
Black Mexican Race (3B) 6.96*** --- 103.66±0.40 44.03  
Palomero Toluqueño Race 5.71±0.11 33(2) ---  --- 2800
Tuxpeño Race (0B) 6.53±0.09 41(2) 84.97** 58.14** 300
Tuxpeño Race (4B) 7.15±0.10 20(1) 94.07±0.35 64.37 300
           
Z. mays ssp. mexicana          
K-67-1 (Chalco) 7.10±0.09 60(2) 117.67±0.76 62.18 2400
K-71-1 (Chalco) 7.71±0.11 38(2) --- ---  2200
           
Z. mays ssp. parviglumis          
K-67-5 (Balsas) 5.88±0.82 20(1) --- ---  1350
K-67-14 (Balsas) 5.76±0.80 19(1) ---  --- 720
K-67-24 (Balsas) 5.85±0.79 44(3) 103.38±0.50 37.41 800
           
Section Luxuriantes          
Z. luxurians  9.83±0.13 20(1) 149.27±0.82 130.14 800
*=Data from Rayburn et al (1985). ** and *** = Expected data.
All lines are from CIMMYT, with the exception of the c-tester line (Leg. by Ing. L. B. Mazoti, LB) and Gaspé flint (Leg. by Dr Patterson, EB).

Total DNA content shows a significant linear positive correlation with TCL (y=3.52 + 14.58 x; r=0.8949) and with TCV (y=81.05 + 20.51 x; r=0.9506). Nuclear DNA value would be more accurately represented by TCV since this cellular parameter does not depend on the degree of chromosomal spiralization.

Rayburn et al. (1985) pointed out that in maize there is a positive correlation among genome size, C+ bands and heterochromatin percentage. This was confirmed by Tito et al. (MNL 64:71, 1990 and TAG, in press) who found a direct relation among number of C-bands in mitosis metaphase, heterochromatin percentage and nuclear DNA value. Tito et al. (l.c.) observed that C-band size is more closely related with C value than the number of C-bands per nucleus is. In the present paper a correspondence is observed between genome size and number and size of C-bands in most of the taxa. The highest proportion of heterochromatic segments is present in Z. luxurians, the species with the highest DNA content (2C=9.83pg). Z. mays ssp. mexicana (2C=7.10-7.65pg) shows conspicuous bands in almost every chromosome of its complement, while the c-tester line of maize (for example) has only four blocks of heterochromatin.

It is interesting to point out that the c-tester line, belonging to Z. mays ssp. mays, did not yield a DNA C value significantly different from that of Z. mays ssp. parviglumis. Nevertheless, the magnitude of the C+ segments is greater in the latter than in the former. This could indicate that heterochromatin is an important factor in the genome size variation, but it is not the only one justifying the differences in DNA content. Depending on the arrangement of the repeated DNA sequences (tandem, separated by unique sequences or moderately repeated, etc.), they could not be detected by the C-banding technique (Tito et al., TAG, in press).

Up to 34 B chromosomes have been detected in maize, with a DNA content increment of 155% (Jones and Rees, 1982). In our study, supernumerary chromosomes increase the genome size from 10 to 16%, 5 being the maximum number observed in the Tuxpeño race from Z. mays ssp. mays.

B chromosomes present in the Black Mexican line and the Tuxpeño race were similar, and both distinguished from autosomes in morphology; they presented neither a visible centromere (being probably telocentric) nor nucleolar organizers. The positive heteropicnosis in mitotic prophase suggests that they are mostly heterochromatic. This should be confirmed through the C-banding method.

It is important to accurately assess the presence of supernumerary chromosomes and their effects on different maize lines both from basic and applied points of view. The reason for such an assertion stems from the knowledge that B chromosomes increase, in some cases, the vegetative period and cell cycle length. They can also affect recombination (through the alteration of chiasmata frequency and localization), growth rate, yield, fertility, etc. (Jones and Rees, 1982). It is thus essential to know, in order to establish predictable breeding programs, whether these observed variations are due to the presence of supernumerary chromosomes or not, and if so, if they behave according to the Mendelian laws.


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